For example, in the manufacture of electronic equipment including the kind of semiconductor device in which a chip is die bonded or a semiconductor device which is subjected to flip-chip bonding, a bonding method with temperature hierarchy is applied, in which connection by soldering at a relatively high temperature using a high temperature solder is performed in a semiconductor device, and then the semiconductor device itself is connected to a substrate by soldering at a relatively low temperature using a low temperature solder having a lower melting point than the high temperature solder.
In performing the solder connection on a high temperature side in the bonding method with temperature hierarchy, a Pb solder was often used in the past, but recently, a Pb-free solder not containing Pb has been frequently used because of concerns about a detrimental effect of Pb on the environment. Examples of a Pb-free solder material which is of interest in the present invention include a solder paste proposed in Japanese Patent Laid-open Publication No. 2002-254194 (Patent Document 1), which includes a mixture of (a) a high melting point metal (or alloy) ball made of a metal such as Cu, Al, Au or Ag, or an alloy containing these metals and (b) a low melting point metal ball made of Sn or In.
When soldering is performed by using the solder paste described in Patent Document 1, as shown schematically in FIG. 6(1), a solder paste 53 including low melting point metal balls 51 made of, for example, Sn, high melting point metal balls 52 made of, for example, Cu and a flux (not shown) reacts by heating, and after soldering, as shown in FIG. 6(2), a connecting part 55 in a state where a plurality of high melting point metal balls 52 are connected to one another with an intermetallic compound, formed between a low melting point metal derived from the low melting point metal ball 51 and a high melting point metal derived from the high melting point metal ball 52, interposed therebetween is formed to give a connection structure in which connecting objects (not shown) are connected to each other by the connecting part 55. A region where an intermetallic compound is produced is shown as an intermetallic compound region 54 in FIG. 6(2).
However, in the above-mentioned connection structure obtained by using a solder paste, when the connecting part is loaded with stress due to distortion resulting from a difference in the coefficient of linear expansion generated by thermal shock or the like, this may result in a problem that a crack is produced in the connecting part, and breaking of wire due to the crack causes an increase in resistance or deterioration of joint strength. This situation will be described in more detail in reference to FIG. 7.
In FIG. 7, a connection structure 60, in which a first connecting object 61 and a second connecting object 62, respectively made of Cu, are connected to each other with a connecting part 63 interposed therebetween, is schematically shown. While the solder paste used in forming the connecting part 63 includes a Cu ball, a Sn ball and a flux as described in Patent Document 1, by heating, the solder paste is put into a state where a plurality of Cu balls 64 are connected to one another between the connecting objects 61 and 62 with a Cu—Sn intermetallic compound, formed between Cu and Sn, interposed between the Cu balls 64.
More specifically, as shown by a heavy line, Cu3Sn layers 65 are formed in such a way as to lie along interfaces between the connecting part 63 and the connecting object 61, and between the connecting part 63 and the connecting object 62, and as to surround the Cu balls 64. Further, a Cu6Sn5 matrix 66 is formed in such a way as to surround the Cu balls 64. Moreover, a Sn matrix 67 derived from the Sn ball remains in the connecting part 63 particularly under common heating conditions in which a long period of heating at a high temperature is not provided.
However, in the connection structure 60 shown in FIG. 7, when the connecting part 63 is loaded with stress due to distortion resulting from a difference in the coefficient of linear expansion generated by thermal shock or the like, stress is apt to be concentrated at a location, for example, interior of the Cu—Sn intermetallic compound such as the Cu3Sn layer 65 and the Cu6Sn5 matrix 66, an interface between the Cu3Sn layers 65, an interface between the Cu6Sn5 matrixes 66, or an interface between the Cu3Sn layer 65 and the Cu6Sn5 matrix 66. Further, the Cu—Sn intermetallic compound itself has mechanically hard and brittle properties. For these reasons, as described above, when the connecting part 63 is loaded with stress, this results in a problem that a crack is easily produced in the connecting part 63.
Patent Document 1: Japanese Patent Laid-open Publication No. 2002-254194